The process for unstable initiation of gene expression via formation of chromosomal tandem duplications of the tryptophan operon will be studied in S. typhimurium. The recA, recB and recC genes and various aspects of recombinational proficiency will be analyzed for their role in the formation and excission of the tandem duplications. The effect of the suppressor of promoter mutations, the supX locus, on the stability of the duplications will also be examined. The effect of chromosomal deletions in altering cotransduction linkage of nearby markers will be analyzed in terms of an interaction between the resulting altered chromosomal nucleotide sequences and the specificity of the bacteriophage encapsulating mechanism.